Mixer circuits are well known in the electronics industry. Mixers have many applications and their use has been extensive. One of the more common applications has been in radio frequency receivers as frequency multipliers or converters. Typically in these types of applications, an incoming modulated RF signal is combined with the signal of a local oscillator (LO) to produce a modulated intermediate frequency (IF) signal. The IF output of the mixer is the difference and sum of the frequencies between the RF and LO frequencies and is then further processed by other known circuits or devices, such as an on-chip active filter.
Many types of mixers are known. One example of a mixer commonly known in the art is the double balanced mixer or the ‘Gilbert Cell’ or ‘Gilbert Mixer’ as shown in FIG. 1. The Gilbert cell is named after its inventor and since then many other mixer topologies have utilized this basic approach. Further modifications and enhancements to the Gilbert cell have been made and are known in the art as will be discussed hereinafter.
The conventional Gilbert cell mixer 10 of FIG. 1 is made up of a RF input stage 20 and a mixer core 30. The RF input stage includes two transistors Q1 and Q2. The base terminal of the first transistor Q1 is coupled to one of two RF signal inputs. The emitter terminal of the first transistor Q1 is coupled to the emitter terminal of the second transistor Q2 and to the current source I1. The collector terminal of the first transistor Q1 is connected to the first pair of differentially connected transistors Q3 and Q4 in the mixer core. The base terminal of the second transistor Q2 is coupled to the second RF signal input. The collector of the second transistor Q2 is coupled to the second pair of differentially connected transistors Q5 and Q6 in the mixer core.
The mixer core is made up of four transistors Q3, Q4, Q5, Q6 coupled as two differentially cross-coupled pairs. The first differentially cross-coupled pair Q3 and Q4 is commonly connected by way of the emitter terminals and further connected to the collector of the first transistor Q1 in the RF input stage. The second differentially cross-coupled pair Q5 and Q6 is also connected by common emitters and further coupled to the collector of the second transistor Q2 in the RF input stage. The base terminal of the first transistor Q3 in the first differential cross-coupled pair of transistors is coupled to the base terminal of the second transistor Q6 of the second differential cross-coupled pair and further coupled to one of two LO input terminals. Similarly, the base terminal of the second transistor Q4 of the first differential cross-coupled pair is coupled to the base terminal of the first transistor Q5 of the second differential cross-coupled pair and further coupled to the second LO input terminal. The collector terminal of the first transistor Q3 of the first differential cross-coupled pair is coupled to the collector terminal of the first transistor Q5 of the second differential cross-coupled pair and further connected to one of two IF output terminals. The collector terminal of the second transistor Q4 of the first differential cross-coupled pair is coupled to the collector terminal of the second transistor Q6 of the second differential cross-coupled pair and further connected to the second IF output terminal.
The operation of the Gilbert cell mixer is well known and need not be elaborated here. In spite of the advantages of the Gilbert cell, there are inherent limitations. For example, the Gilbert cell has limited dynamic range, which is the difference between IP3 (third order intercept point) and noise figure for a given power consumption. The Gilbert mixer's noise figure, linearity, and current drain performance also have been conventionally defined. Consequently, inventions since the introduction of the Gilbert cell have centered on improving various parametric concerns usually dependent on unique mixer applications. One such case is the ‘Micromixer’ which is described in detail in “The MICROMIXER: A Highly Linear Variant of the Gilbert Mixer Using a Bisymmetric Class-AB Input Stage,” by Barrie Gilbert, IEEE Journal of Solid-State Circuits, Vol. 32, No. 9, September 1997 and hereby incorporated by reference.
Referring to FIG. 2, the difference between the Micromixer and the Gilbert mixer of FIG. 1 is apparent. The differential pair in the RF Input section of the Gilbert cell of FIG. 1 has been replaced by a bisymmetric Class AB topology in the Micromixer of FIG. 2. The operation of the Micromixer is also known in the art. This improvement over the Gilbert mixer results in increased mixer performance by way of increased linearity, accurate input impedance, high intermodulation intercepts and nearly unlimited signal capacity. It does not, however, significantly improve noise figure.
Another technique to improve the performance of conventional Gilbert mixers is the use emitter degeneration resistors. This technique, however, introduces resistive thermal noise, which degrades the dynamic range of the differential pair in the RF input section. Yet another technique to improve the performance of the Gilbert mixer involves the use of multi-tanh doublets or triplets as demonstrated in U.S. Pat. No. 6,054,889, “Mixer with Improved Linear Range.” While the use of multi-tanh doublet or triplet approach into a conventional Gilbert mixer improves its performance, it does so at the expense of increasing complexity and loss of valuable chip real estate. Additionally, matching impedance to 50 Ω typically requires impedance transformation at the input.
In other types of mixers, the diode ring and Schottky diode ring mixer for example, the use of baluns, active or passive, increase mixer performance. As demonstrated in U.S. Pat. No. 6,094,570, “Double-balanced Monolithic Microwave Integrated Circuit Mixer” and U.S. Pat. No. 6,078,802, “High Linearity Active Balance Mixer, the use of baluns is known in the art and provide the transfer of RF energy from an unbalanced structure to a balanced structure thereby increasing mixer parametric performance such as linearity and impedance matching for example. Despite advances in integrated circuit fabrication, the use of baluns still take up valuable chip real estate and their use is a compromise in circuit design.
What is needed is a mixer circuit that addresses the known deficiencies in the prior art mixers. Specifically, what is needed is an improved mixer circuit, which provides increased mixer performance by way of a single ended input, and which exhibits a high dynamic range, a low noise figure, and with no off chip differential RF circuit or baluns required.